I know everyone has been waiting, so here it is. Just let me know what you think. We went without the winglets on this design. I might put them on just to see. If people order they need to let us know if they has stock exhaust or aftermarket. This one is for stock and we will supply the exhaust hole. If you have aftermarket then you will have to cut your own.
I got over 1 MPG average driving 60mph-75mph. I also got an improvement from in town driving.

How much are you going to charge? I will probably get one after I get my fswerks exhaust installed. Which will be after I finish building my heat shield for the steeda, so maybe another month or 2. I would probably vinyl it black to match my paint.

Here's how it works. 1 As the air goes under the car it gets caught by the rear bumper. 2 The air under the car moves slower than on top. By using the sheet aluminum it keeps the air from getting caught up under the car and speeds the air back up.

Ok, so here is a stupid question: If this piece of sheet metal can gain 1 MPG, why didn't Ford include it?

I'm not trying to be smart here, but I assume there has got to be some trade-off somehwere...
Is it just cost? Does it cause a build up of heat? Is it more likely to snag something on the road?

My best guess, which should be taken with a huge grain of salt as I have little experience/expertise in all of this, would be cost.

I'm basing this off the Mazda CX-5 Mazda just released. The entire underside of that car has diffuser panels on it to reduce drag and increase MPGs. It is class leading fuel economy and beats out a bunch of mid sized cars too! But they can do that because they charge a pretty penny for that car.

A diffuser, in an automotive context, is a shaped section of the car underbody which improves the car's aerodynamic properties by enhancing the transition between the high-velocity airflow underneath the car and the much slower freestream airflow of the ambient atmosphere. It works by providing a space for the underbody airflow to decelerate and expand so that the boundary between the car's airflow and "external" airflow is less turbulent. It also provides a degree of "wake infill" (the wake being a turbulent area of low pressure that is caused by the passage of the vehicle through the air; this can cause pressure drag).

As the air enters towards the front of the car it accelerates and reduces pressure. There is a second suction peak at the transition of the flat bottom and diffuser. The diffuser then eases this high velocity air back to normal velocity and also helps fill in the area behind the car making the whole underbody a more efficient downforce producing device by reducing drag on the car and increasing downforce.

The aft part of a car underbody can be a diffuser. It works to bring the low pressure air below the car back to the ambient atmospheric pressure without inducing turbulence. It uses Bernoulli's principle, such that the pressure increases while the velocity decreases. Since the pressure below the car is lower than on the side and above the car, downforce is produced if implemented correctly. Injecting the exhaust into the rear diffuser can also help extract the air from below the car. The exhaust gasses effectively energize the boundary layer, helping to raise the pressure of the low-pressure, fast-moving airstream back to the ambient atmospheric pressure at the exit of the diffuser. This helps extract the air more efficiently from the underbody.

Note that the front of the car slows down the air without a diffuser making this the ideal place for an inlet. Instead, a splitter is commonly used here. The splitter serves to reduce the pressure below the front of the car and thereby increase the amount of downforce in that region. The airstream is brought to stagnation above the splitter, by an air dam, causing an area of high pressure. Below the splitter, air is accelerated, as stated above, which causes the pressure to drop and create downforce. Some race cars, such as the Toyota GT-One actually use a proper diffuser in this area to help create more downforce

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